This project examines the nature of the acoustic representations in the lexicons of typically developing (TD) children and children with Specific Language Impairments (SLI). Previous research has suggested that young TD children store words holistically, while older children and adults store words as sequences of phonemes. New evidence suggests, however, that TD children as young as 18 mos. are in fact sensitive to acoustic-phonetic detail in the speech stream. In light of these findings, that school-aged children with SLI may not be sensitive to the acoustic-phonetic details of novel words is even more striking. A series of studies are proposed to investigate sensitivity to acoustic-phonetic detail in TD children and children with SLI. Specifically, the proposed studies are designed to investigate children?s sensitivity to phonotactic frequency - the frequency of sound occurrences and co-occurrences; to examine phonotactic influences on lexical development in both TD and SLI children; and to explore the potential mechanisms implicated in the atypical lexical development of children with SLI. Findings from these studies will extend our understanding of lexical representations in TD children as well as elucidate nonword repetition and novel word learning difficulties seen in children with SLI.